It is confirmed experimentally that with a retarding-field triode, electron oscillations can be maintained whose wave-lengths are determined completely by the external circuit (Gill-Morell oscillations); and that, in addition, oscillations can be obtained with wave-lengths completely independent of the external circuit, provided the latter does not approach resonance with them (Barkhausen-Kurz oscillations). The dependence of the Barkhausen-Kurz oscillations on the valve-dimensions and operating conditions is examined in detail. It is found that for any given set of conditions there are not one, but two possible fundamental wave-lengths: type A, of shorter wave-length, and type B, of longer wave-length, both of which may give harmonics. For any valve of grid radius rg, and at constant anode voltage, characteristic curves λA Vg1/2/rg = FA(ig/i0) and λB Vg1/2/rg = FB(ig/i0) may be obtained, where λA and λB are the wave-lengths of the A- and B-type oscillations, Vg is the grid voltage, and ig/i0 is the degree of saturation of the grid-cathode space. Except for certain limiting conditions, the dependence of FA and FB on valve-dimensions and operating conditions is small, a slight decrease being obtained with decrease of the ratio (grid spacing)/(grid radius), or with decrease of grid voltage. By application of the theory developed in an earlier paper, the shape of the characteristic A curve establishes that the A-type oscillations are due to the periodic transit of electrons between the cathode and virtual cathode. The shape of the characteristic B curve, on the other hand, shows that the B-type oscillations are not due to any such transit of electrons, and leads to the conclusion that they are determined by the resonant circuit formed by the grid-plate capacity in parallel with the equivalent inductance of the electron cloud between these electrodes.